Our present invention relates to a moving coil linear actuator and, more particularly, to a linear actuator of the type which utilizes an electromagnetic force generated by the interaction of a coil with a field of a permanent magnet.
Moving coil actuators, also termed xe2x80x9cvoice coil actuatorsxe2x80x9d, can be utilized to provide direct drive in control systems as servomotors with linear characteristics and for the most part are traditional servo devices since the force which is produced by the interaction of the coil and the field of the permanent magnet is proportional to the current applied to the coil. Such devices can also be referred to as two-terminal devices since the current is applied to the two conductors of a coil and the direction of movement of the coil is a function of the direction of current flow through the latter. Such devices can be used in applications requiring high acceleration, high frequency actuation and a flat force versus displacement characteristic.
The moving coil linear actuator is a direct-drive linear motor and utilizes the principles described in xe2x80x9cThe Straight Attractionxe2x80x9d, Part One, Tony Morcos, Motion Control June 2000, pages 29-33, xe2x80x9cThe Straight Attractionxe2x80x9d, Part Two, Tony Morcos, July/August 2000, Motion Control, pages 24-28, U.S. Pat. No. 5,345,206, U.S. Pat. No. 4,808,955.
In all of these systems, the moving coil linear actuator may suffer from a wasting of air gap space because of a low packing factor of the conductive material of the coil, a greater weight of the unit than may be necessary, problems with flux leakage at an open end of the coil and flux irregularity which may result because of segmentation of the magnets as will be described in greater detail hereinafter.
It is the principal object of the present invention to provide a moving coil linear actuator which is free from at least some of the drawbacks of earlier actuators and has the advantage thereover of being of reduced weight and greater magnetic flux efficiency.
Another object of the invention is to provide an improved moving coil actuator in which there is better utilization of the air gap within the actuator than has been the case heretofore.
Another object of this invention is to provide a moving coil linear actuator which has all of the advantages of prior art actuators but is of reduced weight and improved response in terms of the utilization of the magnetic field.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention by providing a flux cladding at the open end of the coil which reduces flux leakage and, in addition, by increasing the conductor density of the coil.
More particularly, the moving coil actuator of the invention can comprise:
a ferromagnetic core;
a coil axially movable on the core and surrounding the core, the coil being comprised of a multiplicity of closely packed turns of a rectangular cross section conductor;
at least one radially polarized circumferentially continuous ring magnet surrounding the coil and magnetically interacting with the coil upon electrical energization thereof to produce a force axially displacing the coil;
a ferromagnetic sleeve open at one axial end, receiving the ring magnet, and connected to the core at an opposite axial end; and
a flux-confining cladding on the magnet and extending over an end of the magnet at the one axial end.
According to the invention, therefore, the coil provides a conductor of rectangular cross section, which, when wound in a close packed relationship of the turns, can have a packing factor (ratio of conductive material to total coil volume) which is at least 80% and can be as great as 85% or more (preferably at least 90%). Of course, the usual conductor is copper.
It has been found to be advantageous to provide a plurality of the radially polarized circumferentially continuous ring magnets in axially-spaced relationship in the sleeve or even axially contiguous with one another. The circumferential continuity of the ring magnets ensures that flux leakage, as can occur with segmented magnets, is eliminated. The cladding itself for the coil can be magnetic and poled transversely to the polarization of the ring magnet.
It has been found to be advantageous, moreover, to make the core and sleeve assembly of a high permeability ferromagnetic material such as vanadium permadur.
The moving coil linear actuator of the invention thus has the advantage that there is a uniform magnetic field in the air gaps because of the whole ring radial magnetization and flux cladding at the open end. The shape of the core assembly, generated by computer as described below, results in a substantial reduction of weight and hence a reduced need for expensive high-permeability ferromagnetic material such as the vanadium permendur.
The packing factor is also high, as noted, being 85% or better.
The flux focusing which results from the cladding allows radially magnetized magnets which are embedded in the interior of the ferromagnetic sleeve or cylinder, to have an optimum thickness which, in accordance with the graphs provided below, will produce maximum force per unit of ohmic power dissipated in the coil and optimal magnet volume. The magnetic field intensity as a result of flux focussing increases away from the magnets and for a fixed power dissipation, the current density decreases with the winding thickness.
The inner core of the ferromagnetic material is joined at the closed end to the permanent magnet assembly of the sleeve.